Conventional cartridge filters are generally in the form of elongated cylinders open at one end and closed at the other, and are cleaned by a reverse pulse of air. This air is generally emitted from a blow tube into the open end of each cartridge. Airflow control and direction modifiers such as venturis, although widely used, do not substantially improve on the cleaning effectiveness.
A particular difficulty experienced by such cleaning system is that a portion of the cartridge, particularly that portion closest to the source of reverse flowing air, does not experience any cleaning at all, whereas that portion that is cleaned tends to be over cleaned and becomes cumulatively damaged as a result.
Tests conducted on cartridges in reverse pulse cleaning systems have shown that along the internal length of the cartridge, cleaning pulse pressures vary markedly. Observation shows an excessive positive air pressure is applied to part of the filter which over cleans, whilst at other parts (which are generally closest to the reverse pulse source) are subject to a low or even negative pressure, which results in a poor cleaning at those parts.
One possible solution to this difficulty is described in German Specification DE4308286A1 of Bissinger GmbH. In this specification there is disclosed the use of a conical element which is interposed between an outlet nozzle and the inlet to a filter cartridge. DE4308286 teaches that the effect of the cone is to produce a sufficient pressure to be built up along the entire length of the filter element. This pressure produces a reverse air flow through the filter element to dislodge particles on the outside of the filter cartridge element.
A conical element similar to DE4308286 has been shown in tests conducted by the applicant to be ineffective as the minimum nozzle orifice diameter increases and the air flow rate increases.
It is unknown as to why such a result occurs, however it does cause a difficulty to the filter cleaning industry in that the cone of DE4308286 is not useful under the current range of conditions that are utilised in the filter cleaning industry.